WHO Pesticide Evaluation Scheme Research Articles

Physical durability: are bed nets getting any stronger?

BackgroundFor at least a decade, concerns have been raised about the physical durability of insecticide-treated nets (ITNs) and their ability to remain in good condition for at least three years. To discover if the resistance to damage (RD) of ITNs has improved or not, the RD scores of ITNs sampled in 2013 and 2020 were compared.MethodsThe RD scores and disaggregated textile performance data for nine ITNs recommended by the WHO pesticide evaluation scheme (WHOPES) measured in 2013 were compared with WHO-prequalified ITNs sampled in 2020. This included assessment of newer ITNs not available in 2013, to determine the extent to which product development has led to performance improvements across all available ITNs in the intervening years.ResultsThe resistance to damage of ITNs has not generally improved from 2013 to 2020, and in some cases performance is worse. The average RD score of comparable ITNs brands decreased from 40 in 2013 to 36 in 2020. Of the nets available in 2020, only two of the twenty-four ITN products tested achieved an RD score of > 50, while six ITNs had very low RD scores of < 30, highlighting a serious inherent, and literal weakness in many WHO-prequalified ITNs.ConclusionsThe long-term physical durability of ITN products cannot be expected to improve while their resistance to damage remains so low, and major upgrades to the performance standards of textile materials used to make ITNs, as well as incentives to develop stronger ones are urgently required.

Evaluation of Yahe\xae and Panda\xae 2.0 long-lasting insecticidal nets against wild pyrethroid-resistant Anopheles gambiae s.l. from C\xf4te d\u2019Ivoire: an experimental hut trial

BackgroundLong-lasting insecticidal nets (LLINs) have played an important role in reducing the global malaria burden since 2000. They are a core prevention tool used widely by people at risk of malaria. The Vector Control Prequalification mechanism of the Word Health Organization (WHO-Vector Control PQ) established the testing and evaluation guidelines for LLINs before registration for public use. In the present study, two new brands of deltamethrin-impregnated nets (Yahe® LN and Panda® Net 2.0) were evaluated in an experimental hut against wild pyrethroid-resistant Anopheles gambiae s.l. in M’Bé nearby Bouaké, central Côte d’Ivoire.MethodsThe performance of Yahe® LN and Panda® Net 2.0 was compared with that of PermaNet 2.0, conventionally treated nets (CTN), and untreated net to assess the blood-feeding inhibition, deterrence, induced exophily, and mortality.ResultsCone bioassay results showed that Panda® Net 2.0, PermaNet 2.0 and Yahe® LN (both unwashed and washed 20 times) induced > 95% knockdown or > 80% mortality of the susceptible Anopheles gambiae Kisumu strain. With the pyrethroid-resistant M’Bé strain, mortality rate for all treated nets did not exceed 70%. There was a significant reduction in entry and blood feeding (p < 0.05) and an increase in exophily and mortality rates (p < 0.05) with all treatments compared to untreated nets, except the CTNs. However, the personal protection induced by these treated nets decreased significantly after 20 washes. The performance of Panda® Net 2.0 was equal to PermaNet® 2.0 in terms of inhibiting blood feeding, but better than PermaNet® 2.0 in terms of mortality.ConclusionThis study showed that Yahe® LN and Panda® Net 2.0 met the WHO Pesticide Evaluation Scheme (WHOPES) criteria to undergo phase III trial at the community level. Due to an increasing spread and development of pyrethroid resistance in malaria vectors, control of malaria transmission must evolve into an integrated vector management relying on a large variety of efficient control tools.Graphical

Species composition, blood meal sources and insecticide susceptibility status of Culex mosquitoes from Jimma area, Ethiopia

Different species of Culex mosquitoes are present in high numbers in many other countries in addition to sub-Saharan Africa and Asian countries. Culex mosquitoes are serious nuisance which also transmit a range of pathogens including several viruses such as West Nile virus, Saint Louis encephalitis, and Rift Valley fever, as well as parasites such as avian malaria, and filarial worms. In Ethiopia, unlike Anopheles mosquitoes, little effort was given to study habitat, species composition, blood meal sources and insecticide susceptibility status of Culex mosquitoes. Therefore, this study aimed to assess the species composition, their blood meal source and insecticide susceptibility status of Culex mosquitoes to some of selected insecticides in Jimma town southwest Ethiopia. Culicine mosquito larvae were collected using a standard dipper (by dipping) from a range of breeding sites and reared to adults. Species identification was carried out using standard keys. Bioassay tests were performed on adults to assess the susceptibility of Culex mosquitoes to insecticide-impregnated papers with Dichlorodiphenyltrichloroethane (DDT (4%), mlathion (5%), bendiocarb (0.1%), propoxur (0.1%), deltamethrin (0.05%) and pirimiphos-methyl (0.25) following World Health Organization Pesticide Evaluation Scheme (WHOPES) guideline. Moreover, 184 blood fed (BF) Culex mosquitoes were collected using aspirator from indoor and outdoor resting and assayed to assess blood meal sources using Enzyme-Linked Immunosorbent Assay (ELISA). The result of the study showed that among the collected Culex moquitoes, two species were identified as Culex quinquefasciatus and Cx antennatus whereas the remaining one could not be identified to species level. Culex mosquitoes were found to be resistant to DDT, malathion, bendiocarb, propoxur, and deltamethrin whereas susceptible to pirimiphos-methyl. The blood meal source analysis using Enzyme-Linked Immunosorbent Assay (ELISA) showed higher blood source of human (33.2%) than bovine (15.2%). Thus, the observed resistance to the most of the insecticides coupled with higher human blood meal source calls further studies to be carried out in Culex mosquito populations of Ethiopia.

Development of gel for Aedes aegypti repellent with combination of cinnamon oil (Cinnamomum burmannii Blume) and fennel oil (Foeniculum vulgare Mill)

Background/Aim: Aim of this study was to develop efficient mosquito repellent by combining fennel oil (Foeniculum vulgare Mill) and cinnamon oil (Cinnamomum burmannii Blume) in a gel preparation form. Methods: Effectiveness of each oil and its combination as viral repellents was tested by the World Health Organization's Pesticide Evaluation Scheme (WHOPES) method. Repellent was prepared in gel form. Evaluation of gel preparation included organoleptic properties, homogeneity, pH, viscosity, flow properties, acute skin irritation test and effectiveness test as mosquito repellent. Results: The mosquito protection test for cinnamon oil showed that cinnamon oil was most effective at concentration of 15 % (96.85 %), and fennel oil at 24 % concentration (79.26 %). Within 6 hours, gel made of 24 % fennel oil and 15 % cinnamon oil combination gave protection against 53.49 % mosquitoes. Conclusion: The combination of cinnamon oil and fennel oil can be formulated into gel form with satisfying physical and chemical characteristics and effectiveness against Aedes aegypti mosquitoes for 6 hours.

Evaluation of MAGNet, a long-lasting insecticidal mosquito net against Anopheles fluviatilis in experimental huts in India

BackgroundMAGNet LN is a wash resistant long-lasting insecticidal (polyethylene) net (LLIN) in which the alpha-cypermethrin insecticide is incorporated within the 150 denier high density polyethylene monofilaments of the nets. The bio-efficacy of MAGNet LN was reported to be high even after 25 washes. The LN met the WHO criteria of Phase I evaluation and obtained recommendation from the World Health Organization Pesticide Evaluation Scheme (WHOPES) for Phase II trial. For registration of the LN in India, the current study was conducted to evaluate its efficacy after 20 or 25 washes compared to negative control (untreated net) and positive control (Duranet LN) in experimental huts against a wild, free flying pyrethroid susceptible population of Anopheles fluviatilis in terms of deterrence, blood-feeding inhibition, mortality and induced exophily.MethodsThe evaluation was carried out in six experimental huts located at Kandhaguda village in Malkangiri district, Odisha state following the WHO guidelines.ResultsThe study showed that 25 times washed MAGNet LN produced 100% mortality in cone bioassays before and after hut evaluation. MAGNet washed 25 times did not differ significantly from all other treated nets in terms of deterring hut entry, induced exophily, blood feeding inhibition and causing mortality of An. fluviatilis.ConclusionsMAGNet LN showed extended wash resistance retaining its bio-efficacy up to 25 washes and met the WHOPES requirement of passing Phase II evaluation.

Sensibilité des populations d’ Aedes Aegypti vis-à-vis des organochlorés, pyréthrinoïdes et des carbamates dans la commune de Natitingou au Nord-Est du Bénin

Insecticide treatments for crop protection have often be cited as the main factor of the resistance selection of Aedes aegypti. This is however the main vector dengue fever. To verify this hypothesis, bioassay tests was performed on adults collected from the field (2 urban and 2 rural areas) at Natitingou to assess the susceptibility of dengue vectors to insecticideimpregnated papers (permethrin 0.75%, delthamethrin 0.05%, lambdacyalothrin 0.05%, DDT 4%, and bendiocarb 0.1%) following WHO Pesticide Evaluation Scheme. Results from this study showed that Ae. Aegypti populations were resistant to DDT and pyrethroids with 12% and 38% respectively as mean of mortality despite the areas of mosquitoes collection. However, they were fully susceptible to bendiocarb. Our study showed that Aedes aegypti populations had developed resistance to organochlorine and pyrethroids, but are susceptible to carbamate. The low mortality rate recorded in all populations of Ae. Aegypti with bendiocarb is encouraging in regards to the use of this insecticide as an alternative in case of outbreak of dengue fever in this district of Benin.

Bio-efficacy of LifeNet, a deltamethrin incorporated long-lasting insecticidal net, as assessed in experimental huts against Anopheles fluviatilis, a major malaria vector in east-central India

LifeNet, a deltamethrin incorporated long-lasting insecticidal (polypropylene) net (LLIN), was qualified by the World Health Organization Pesticide Evaluation Scheme (WHOPES) for Phase-II trial in India. The purpose of this trial was to assess the bio-efficacy of unwashed and 20 and 30 times washed LifeNet in comparison to the nets conventionally treated with deltamethrin against the natural population of Anopheles fluviatilis, a major malaria vector, in terms of deterring hut entry, inhibiting blood feeding, inducing exophily and causing mortality. The trial was carried out in six experimental huts constructed at Kandhaguda village in Malkangiri district, Odisha State. The efficacy of unwashed and washed (20 or 30 times) LifeNet was compared with untreated polypropylene and conventionally treated (with deltamethrin) polyester net washed to just before exhaustion or washed 20 times. The study showed a significant reduction of entry (treatment: 1.61–4.78; control: 7.61 per hut) and an increase in exit (50.7–64.4% and 39.1%) of An. fluviatilis in the treated arms compared to the control arm (untreated net) (P < 0.05). Blood feeding rates reduced in treated arms (20.7–68.0%) compared to the control (80.3%) (P < 0.05). Total mortality was significantly higher in LifeNet arms (73.8–98.3%) than the control (2.2%) (P < 0.05). After 30 washes, the active ingredient (AI) retention in LifeNet was 62%. Performance of the three LifeNet arms against the susceptible population of An. fluviatilis met the WHO efficacy criteria of Phase II evaluation for LLINs.

Assessing bed net damage: comparisons of three measurement methods for estimating the size, shape, and distribution of holes on bed nets

BackgroundMeasuring the physical condition of long-lasting insecticidal nets (LLINs) under field conditions is of great importance for malaria control programmes to guide decisions on how frequently to replace LLINs. Current guidelines by the World Health Organization Pesticide Evaluation Scheme (WHOPES) propose a proportionate hole index (pHI) for assessing LLIN condition by counting the number of holes the size of a thumb, fist, head, and larger than a head. However, this method does not account for irregular hole shapes or exact hole sizes which could result in inaccurate decisions about when to replace LLINs.MethodsLLINs were collected during a 2013 health facility-based malaria case control study in Machinga District, Malawi. To evaluate the accuracy of the pHI, the physical condition of 277 LLINs was estimated by the WHOPES method and then compared with two more thorough measurement methods: image analysis of digital photographs of each LLIN side; and for 10 nets, ruler measurements of the length, width, and location of each hole. Total hole counts and areas per net were estimated by each method, and detailed results of hole shapes and composite pictures of hole locations were generated using image analysis.ResultsThe WHOPES method and image analysis resulted in similar estimates of total hole counts, each with a median of 10 (inter-quartile range (IQR) 4–24 and 4–23, respectively; p = 0.004); however, estimated hole areas were significantly larger using the WHOPES method (median 162 cm2, IQR 28–793) than image analysis (median 13 cm2, IQR 3–101; p < 0.0001). The WHOPES method classified fewer LLINs in ‘good condition’ compared to image analysis (42% vs 74%). The ruler method detected significantly more holes than image analysis did (p = 0.002) in 10 LLINs; however, total hole area was not significantly different (p = 0.16). Most holes were not circular but roughly 2–5 times longer in one direction. The lower quarter of LLIN sides was found to have the most holes.ConclusionsThe WHOPES method overestimated total hole area, likely because holes are elongated rather than circular, suggesting further adjustments to the pHI formula may be warranted when considering LLIN replacement strategies.

Phase III evaluation of the insecticidal efficacy and durability of a deltamethrin-treated polypropylene long-lasting net LifeNet®, in comparison with long-lasting nets made from polyester and polyethylene: study protocol.

BackgroundUniversal coverage of long-lasting insecticidal nets (LNs) made from polyester or polyethylene fibres has been adopted as the standard of care to control malaria among at-risk populations. To obtain a WHO recommendation, LNs must undergo prospective monitoring of insecticidal efficacy against mosquito vectors over 3 years of household use. The retention of bioefficacy and physical durability of a LN is influenced by net usage practices, textile polymer material and insecticide treatment technology. Fabric durability is the critical factor which determines the interval required between LN replacement campaigns. To investigate factors known to affect LN durability and bioefficacy, we describe a three-arm WHO Pesticide Evaluation Scheme (WHOPES) Phase III evaluation of a LN made uniquely from polypropylene (LifeNet®, Bayer CropScience) compared to standard LNs made from polyester and polyethylene, all treated with deltamethrin, over 3 years of use.MethodsThis is a prospective three-arm household randomized, equivalence trial of LNs in Tanzania, with nets as the unit of observation. Equal numbers of houses will be randomized to receive deltamethrin-treated polypropylene, polyester or polyethylene LNs; all sleeping spaces in a given household will be provided with one type of net. Bioefficacy (insecticidal activity against mosquitoes), insecticide content of net fibres, and fabric integrity (number, location and size of holes) will be measured every 6 months, using WHO cone or tunnel bioassays, chemical analysis and calculation of hole index, respectively. A cohort of LNs will be surveyed annually to assess survivorship (median LN survival time) and cumulative loss of fabric integrity. Field durability outcomes will be compared with laboratory strength tests.DiscussionThis is the first trial to compare the relative durability of three LNs each made from a different textile polymer, treated with the same insecticide, in the same community side-by-side over 3 years of use. Trial findings will 1) guide global health organizations on procurement policy and the type of textile polymer which maximizes the interval between LN replacement campaigns, and 2) stimulate manufacturers to improve product performance and development of longer lasting polymers. A full WHO recommendation may be granted to LifeNet® upon successful Phase III completion.

Investigating a Non-Mesh Mosquito Net Among Outdoor Sleeping Nomadic Communities in Kenya.

Rising reports of exophagic malaria vectors make even more pressing the need for alternatives to traditional, mesh, long-lasting insecticidal nets (LLINs) designed for indoor sleeping and often inadequate in the protection of outdoor-sleeping populations. This study tests and evaluates the retention, utilization, and durability of novel, non-mesh nets designed for outdoor use. Longitudinal, cross-sectional surveys were conducted, the physical condition of nets was assessed, and bio-efficacy and insecticide content were tested. At 22 months, retention was 98.0%; 97.1% of nets fell within the World Health Organization (WHO) category of being in “good” condition; none were in the “torn” category. At 18 months post-distribution, 100% of nets had at least WHO Pesticide Evaluation Scheme (WHOPES)-acceptable levels of insecticide, this proportion was 66.7% at 22 months. This novel mosquito net has the potential to provide a durable and context-specific tool to prevent malaria among traditionally hard-to-protect and highly vulnerable populations.

Bed Net Durability Assessments: Exploring a Composite Measure of Net Damage.

BackgroundThe durability of Long Lasting Insecticidal Nets (LLINs) in field conditions is of great importance for malaria prevention and control efforts; however, the physical integrity of the net fabric is not well understood making it challenging to determine overall effectiveness of nets as they age. The 2011 World Health Organization Pesticide Evaluation Scheme (WHOPES) guidelines provide a simple, standardized method using a proportional hole index (PHI) for assessing net damage with the intent to provide national malaria control programs with guidelines to assess the useful life of LLINS and estimate the rate of replacement.MethodsWe evaluated the utility of the PHI measure using 409 LLINs collected over three years in Nampula Province, Mozambique following a mass distribution campaign in 2008. For each LLIN the diameter and distance from the bottom of the net were recorded for every hole. Holes were classified into four size categories and a PHI was calculated following WHOPES guidelines. We investigate how the size, shape, and location of holes influence the PHI. The areas of the WHOPES defined categories were compared to circular and elliptical areas based on approximate shape and actual measured axes of each hole and the PHI was compared to cumulative damaged surface area of the LLIN.ResultsThe damaged area of small, medium, large, and extra-large holes was overestimated using the WHOPES categories compared to elliptical areas using the actual measured axes. Similar results were found when comparing to circular areas except for extra-large holes which were underestimated. (Wilcoxon signed rank test of differences p< 0.0001 for all sizes). Approximating holes as circular overestimated hole surface area by 1.5 to 2 times or more. There was a significant difference in the mean number of holes < 0.5 cm by brand and there were more holes of all sizes on the bottom of nets than the top. For a range of hypothetical PHI thresholds used to designate a “failed LLIN”, roughly 75 to 80% of failed LLINs were detected by considering large and extra-large holes alone, but sensitivity varied by brand.ConclusionsFuture studies may refine the PHI to better approximate overall damaged surface area. Furthermore, research is needed to identify whether or not appropriate PHI thresholds can be used to deem a net no longer protective. Once a cutoff is selected, simpler methods of determining the effective lifespan of LLINs can help guide replacement strategies for malaria control programs.

Evaluation of ICON Maxx, a long-lasting treatment kit for mosquito nets: experimental hut trials against anopheline mosquitoes in Tanzania.

BackgroundInsecticide-treated nets are the primary method of preventing malaria. To remain effective, the pyrethroid insecticide must withstand multiple washes over the lifetime of the net. ICON® Maxx is a ‘dip-it-yourself’ kit for long-lasting treatment of polyester nets. The twin-sachet kit contains a slow-release capsule suspension of lambda-cyhalothrin plus binding agent. To determine whether ICON Maxx meets the standards required by the World Health Organization Pesticide Evaluation Scheme (WHOPES), the efficacy and wash fastness of ICON Maxx was evaluated against wild, free-flying anopheline mosquitoes.MethodsICON Maxx was subjected to bioassay evaluation and experimental hut trial against pyrethroid-susceptible Anopheles gambiae, Anopheles arabiensis and Anopheles funestus. Mosquito mortality, blood feeding inhibition and personal protection were compared between untreated nets, conventional lambda-cyhalothrin treated nets (CTN) washed either four times (cut-off threshold) or 20 times, and ICON Maxx-treated nets either unwashed or washed 20 times.ResultsIn bioassay, ICON Maxx demonstrated superior wash resistance to the CTN. In the experimental hut trial, ICON Maxx killed 75 % of An. funestus, 71 % of An. gambiae and 47 % of An. arabiensis when unwashed and 58, 66 and 42 %, respectively, when 20 times washed. The CTN killed 52 % of An. funestus, 33 % of An. gambiae and 30 % of An. arabiensis when washed to the cut-off threshold of four washes and 40, 40 and 36 %, respectively, when 20 times washed. Percentage mortality with ICON Maxx 20 times washed was similar (An. funestus) or significantly higher (An. gambiae, An. arabiensis) than with CTN washed to the WHOPES cut-off threshold. Blood-feeding inhibition with ICON Maxx 20 times washed was similar to the CTN washed to cut-off for all three species. Personal protection was significantly higher with ICON Maxx 20 times washed (66-79 %) than with CTN washed to cut-off (48-60 %).ConclusionsNets treated with ICON Maxx and washed 20 times met the approval criteria set by WHOPES for Phase II trials in terms of mortality and blood-feeding inhibition. This finding raises the prospect of conventional polyester nets and other materials being made long-lastingly insecticidal through simple dipping in community or home, and thus represents a major advance over conventional pyrethroid treatments.

The impact of different sprayable surfaces on the effectiveness of indoor residual spraying using a micro encapsulated formulation of lambda-cyhalothrin against Anopheles gambiae s.s.

BackgroundThe type of sprayable surface impacts on residual efficacy of insecticide used in indoor residual spraying (IRS). However, there is limited data on common types of wall surfaces sprayed in Zanzibar and mainland Tanzania where IRS began in 2006 and 2007 respectively. The study investigated residual efficacy of micro-encapsulated lambda-cyhalothrin sprayed on common surfaces of human dwellings and domestic animal shelters in Zanzibar and mainland Tanzania.MethodsAn experimental hut was constructed with different types of materials simulating common sprayable surfaces in Zanzibar and mainland Tanzania. Surfaces included cement plastered wall, mud-daub, white-wash, wood, palm-thatch, galvanized iron-sheets, burnt-bricks, limestone and oil-paint. The World Health Organization (WHO) procedure for IRS was used to spray lambda-cyhalothrin on surfaces at the dose of 20–25 mg/m2. Residual efficacy of insecticide was monitored through cone bioassay using laboratory-reared mosquitoes; Kisumu strain (R–70) of Anopheles gambiae ss. Cone bioassay was done every fortnight for a period of 152 days. The WHO Pesticide Evaluation Scheme (WHOPES) threshold (80% mortality) was used as cut-off point for acceptable residual efficacy.ResultsA total of 5,800 mosquitoes were subjected to contact cone bioassay to test residual efficacy of lambda-cyhalothrin. There was a statistically significant variation in residual efficacy between the different types of wall surfaces (r = 0.24; p < 0.001). Residual efficacy decreased with increasing pH of the substrate (r = −0.5; p < 0.001). Based on WHOPES standards, shorter residual efficacy (42-56 days) was found in wall substrates made of cement, limestone, mud-daub, oil paint and white wash. Burnt bricks retained the residual efficacy up to 134 days while galvanized iron sheets, palm thatch and wood retained the recommended residual efficacy beyond 152 days.ConclusionThe study revealed a wide variation in residual efficacy of micro encapsulated formulation of lambda-cyhalothrin across the different types of wall surfaces studied. In areas where malaria transmission is bimodal and wall surfaces with short residual efficacy comprise > 20% of sprayable structures, two rounds of IRS using lambda-cyhalothrin should be considered. Further studies are required to investigate the impact of sprayable surfaces on residual efficacy of other insecticides commonly used for IRS in Zanzibar and mainland Tanzania.

Aquatain® Mosquito Formulation (AMF) for the control of immature Anopheles gambiae sensu stricto and Anopheles arabiensis: dose-responses, persistence and sub-lethal effects.

BackgroundPersistent monomolecular surface films could benefit larval source management for malaria control by reducing programme costs and managing insecticide resistance. This study evaluated the efficacy of the silicone-based surface film, Aquatain® Mosquito Formulation (AMF), for the control of the Afrotropical malaria vectors, Anopheles gambiae sensu stricto and Anopheles arabiensis in laboratory dose–response assays and standardized field tests.MethodsTests were carried out following guidelines made by the World Health Organization Pesticide Evaluation Scheme (WHOPES). Sub-lethal effects of AMF were evaluated by measuring egg-laying and hatching of eggs laid by female An. gambiae s.s. that emerged from habitats treated with a dose that resulted in 50% larval mortality in laboratory tests.ResultsBoth vector species were highly susceptible to AMF. The estimated lethal doses to cause complete larval mortality in dose–response tests in the laboratory were 1.23 (95% confidence interval (CI) 0.99-1.59) ml/m2 for An. gambiae s.s. and 1.35 (95% CI 1.09-1.75) ml/m2 for An. arabiensis. Standardized field tests showed that a single dose of AMF at 1 ml/m2 inhibited emergence by 85% (95% CI 82-88%) for six weeks. Females exposed as larvae to a sub-lethal dose of AMF were 2.2 times less likely (Odds ratio (OR) 0.45, 95% CI 0.26-0.78) to lay eggs compared to those from untreated ponds. However, exposure to sub-lethal doses neither affected the number of eggs laid by females nor the proportion hatching.ConclusionAMF provided high levels of larval control for a minimum of six weeks, with sub-lethal doses reducing the ability of female mosquitoes to lay eggs. The application of AMF provides a promising novel strategy for larval control interventions against malaria vectors in Africa. Further field studies in different eco-epidemiological settings are justified to determine the persistence of AMF film for mosquito vector control and its potential for inclusion in integrated vector management programmes.

Proposed use of spatial mortality assessments as part of the pesticide evaluation scheme for vector control

BackgroundThe WHO Pesticide Evaluation Scheme to evaluate the efficacy of insecticides does not include the testing of a lethal effect at a distance. A tool was developed to evaluate the spatial mortality of an insecticide product against adult mosquitoes at a distance under laboratory and field conditions. Operational implications are discussed.MethodsInsecticide paint, Inesfly 5A IGR™, containing two organophosphates (OPs): chlorpyrifos and diazinon, and one insect growth regulator (IGR): pyriproxyfen, was the product tested. Laboratory tests were performed using “distance boxes” with surfaces treated with one layer of control or insecticide paint at a dose of 1 kg/6 sq m. Field tests were conducted up to 12 months in six experimental huts randomly allocated to control or one or two layers of insecticide paint at 1 kg/6 sq m. All distance tests were performed using reference-susceptible strains of Anopheles gambiae and Culex quinquefasciatus left overnight at a distance of 1 m from control or treated surfaces.ResultsAfter an overnight exposition at distances of 1 m, field and laboratory evaluations at 0 months after treatment (T0) yielded 100% mortality rates on surfaces treated with one layer at 1 kg/6 sq m against susceptible strains of An. gambiae and Cx. quinquefasciatus. Testing for long-term efficacy in the field gave mortality rates of 96-100% after an overnight exposition at a distance of 1 m for up to 12 months in huts where a larger volume was treated (walls and ceilings) with one or two layers of insecticide paint.ConclusionA comprehensive evaluation of the full profile of insecticide products, both upon contact and spatially, may help rationalize vector control efforts more efficiently. Treating a large enough volume may extend a product’s mortality efficacy in the long-term, which contact tests would fail to assess. It is hereby proposed to explore the development of cost effective methods to assess spatial mortality and to include them as one additional measurement of insecticide efficacy against mosquitoes and other arthropod vectors in WHOPES Phase I and Phase II studies.

Evaluation of the long-lasting insecticidal net Interceptor LN: laboratory and experimental hut studies against anopheline and culicine mosquitoes in northeastern Tanzania

BackgroundLong lasting insecticidal nets (LN) are a primary method of malaria prevention. Before new types of LN are approved they need to meet quality and efficacy standards set by the WHO Pesticide Evaluation Scheme. The process of evaluation has three phases. In Phase I the candidate LN must meet threshold bioassay criteria after 20 standardized washes. In Phase II washed and unwashed LNs are evaluated in experimental huts against wild, free flying anopheline mosquitoes. In Phase III the LN are distributed to households in malaria endemic areas, sampled over three years of use and tested for continuing insecticidal efficacy. Interceptor® LN (BASF Corporation, Germany) is made of polyester netting coated with a wash resistant formulation of alpha-cypermethrin.MethodsInterceptor LN was subjected to bioassay evaluation and then to experimental hut trial against pyrethroid-susceptible Anopheles gambiae and An. funestus and resistant Culex quinquefasciatus. Mosquito mortality, blood feeding inhibition and personal protection were compared between untreated nets, conventional alpha-cypermethrin treated nets (CTN) washed 20 times and LNs washed 0, 20 and 30 times.ResultsIn Phase I Interceptor LN demonstrated superior wash resistance and efficacy to the CTN. In the Phase II hut trial the LN killed 92% of female An. gambiae when unwashed and 76% when washed 20 times; the CTN washed 20 times killed 44%. The LN out-performed the CTN in personal protection and blood-feeding inhibition. The trend for An. funestus was similar to An. gambiae for all outcomes. Few pyrethroid-resistant Cx. quinquefasciatus were killed and yet the level of personal protection (75-90%) against Culex was similar to that of susceptible An. gambiae (76-80%) even after 20 washes. This protection is relevant because Cx. quinquefasciatus is a vector of lymphatic filariasis in East Africa. After 20 washes and 60 nights’ use the LN retained 27% of its initial insecticide dose.ConclusionsInterceptor LN meets the approval criteria set by WHO and is recommended for use in disease control against East African vectors of malaria and filariasis. Some constraints associated with the phase II evaluation criteria, in particular the washing procedure, are critically reviewed.

Residual activity and integrity of PermaNet® 2.0 after 24 months of household use in a community randomised trial of long lasting insecticidal nets against visceral leishmaniasis in India and Nepal

The World Health Organization (WHO) recommends several brands of long lasting insecticidal net (LN) for protection against insect vectors but also advises national programmes to monitor and evaluate performance under local conditions to help them select the most suitable LN for their setting. During the course of a community randomised trial of LNs against visceral leishmaniasis in northern India and Nepal, opportunity arose to assess the efficacy of PermaNet 2.0 (Vestergaard-Frandsen, Denmark) after two years of use against sandfly vectors. Between 63% (India) and 78% (Nepal) of LNs became holed over the course of two years, deltamethrin residues fell from 55 mg/m(2) to an average of 11.6 mg/m(2) (India) and 27.9 mg/m(2) (Nepal), but on the basis of bioassay criteria all LNs tested still met the WHO Pesticide Evaluation Scheme standard for LN effectiveness. Nets had on average only been washed 2.5 times (India) and 0.6 times (Nepal) by householders over the course of two years. The loss of insecticide was attributed to factors which had little or nothing to do with washing, such as handling, friction and torsion during daily use. Under conditions pertaining in this region of south Asia, and for two years at least, this brand of net continues to meet the criteria established by WHO for LNs.

Durability associated efficacy of long-lasting insecticidal nets after five years of household use

BackgroundLong-lasting insecticidal nets (LLINs) have been strongly advocated for use to prevent malaria in sub-Saharan Africa and have significantly reduced human-vector contact. PermaNet® 2.0 is among the five LLINs brands which have been given full approval by the WHO Pesticide Evaluation Scheme (WHOPES). The LLINs are expected to protect the malaria endemic communities, but a number of factors within the community can affect their durability and efficacy. This study evaluated the durability, efficacy and retention of PermaNet® 2.0 after five years of use in a Tanzanian community.MethodTwo to three day- old non blood-fed female mosquitoes from an insectary susceptible colony (An. gambiae s.s, this colony was established at TPRI from Kisumu, Kenya in 1992) and wild mosquito populations (An. arabiensis and Culex quinquefasciatus) were used in cone bioassay tests to assess the efficacy of mosquito nets.FindingsThe knockdown effect was recorded after three minutes of exposure, and mortality was recorded after 24 hours post-exposure. Mortality of An. gambiae s.s from insectary colony was 100% while An. arabiensis and Cx.quinquefasciatus wild populations had reduced mortality. Insecticide content of the new (the bed net of the same brand but never used before) and used PermaNet® 2.0 was determined using High Performance Liquid Chromatography (HPLC).ConclusionThe results of this study suggest that, in order to achieve maximum protection against malaria, public health education focusing on bed net use and maintenance should be incorporated into the mass distribution of nets in communities.

Plant-based insect repellents: a review of their efficacy, development and testing.

Plant-based repellents have been used for generations in traditional practice as a personal protection measure against host-seeking mosquitoes. Knowledge on traditional repellent plants obtained through ethnobotanical studies is a valuable resource for the development of new natural products. Recently, commercial repellent products containing plant-based ingredients have gained increasing popularity among consumers, as these are commonly perceived as “safe” in comparison to long-established synthetic repellents although this is sometimes a misconception. To date insufficient studies have followed standard WHO Pesticide Evaluation Scheme guidelines for repellent testing. There is a need for further standardized studies in order to better evaluate repellent compounds and develop new products that offer high repellency as well as good consumer safety. This paper presents a summary of recent information on testing, efficacy and safety of plant-based repellents as well as promising new developments in the field.

Africa's largest long-lasting insecticide-treated net producer: lessons from A to Z Textiles

BackgroundField trials have demonstrated the efficacy of insecticide-treated nets, and the WHO has recently endorsed a shift toward Long-Lasting Insecticide Treated nets (LLINs) due to factors such as reduced distribution costs. However, the need for LLINs poses several challenges. Is it possible to manufacture LLINs in large quantities in the African continent, where malaria is most endemic? When production is located in low-income countries, what role is played by local funding and employment, scaling up manufacturing, and partnerships? What factors influence availability and pricing?DiscussionA case study of A to Z Textiles was undertaken to answer the question of how large-scale production of LLINs can occur in a low income setting. One of the largest sources of bed nets for Africa, A to Z Textiles is Africa-based, and its Tanzanian operations have a production capacity of 30 million LLINs per year, along with full WHO recommendation for its nets. Our analysis is based on semi-structured interviews with key informants familiar with A to Z, site visits in Tanzania, and literature reviews.This paper discusses the history and current status of A to Z Textiles, identifies the factors that led to its success, and suggests policy considerations that could support similar initiatives in the future. Local funding, scaling up manufacturing, technology transfer, and partnerships all played important roles in A to Z’s ascent, as did perceived benefits of local employment and capacity-building. Regulatory issues and procurement rules acted as barriers. A to Z cost-effectively manufactures high-quality LLINs where malaria is most endemic.SummaryWith a production capacity of 30 million LLINs per year, and full WHOPES (WHO Pesticide Evaluation Scheme) certification, A to Z Textiles demonstrates how key health goods can be successfully produced in the low-income countries that use them. Its example may be instructive and of high interest to readers in the malaria community, especially in developing countries, and to those who wish to support or partner with efforts by developing countries to build their health innovation capacity.